Each of us harbors a distinct collection of trillions of microbes resulting in a 'supraorganism'where the number of microbial cells exceeds the number of human cells by an estimated order of magnitude. Our gut contains the vast majority of these microbes. The sources of microbial community stability, and the mechanisms by which co-evolution of an individual's immune system and microbiota contributes to the symbiotic relationship are poorly understand: the answers should provide important clues about how the microbiota contributes to our health, how perturbations in microbial ecology arise, how such perturbations produce certain pathologic states (e.g., infectious diarrheas;inflammatory bowel diseases) and how new strategies can be developed for intentionally manipulating the representation of components of the microbiota for therapeutic benefit. I have developed a simplified model of the human gut microbiota in gnotobiotic Rag1-/- mice, where the diversity of the gut microbiota is reduced to one species (Bacteroides thetaiotaomicron, a prominent sequenced member of the normal human gut microbiota), and the repertoire of the adaptive immune system to one immunoglobulin. I have characterized the specificity of this unique immunoglobulin A (IgA), which was naturally primed by colonization of germfree mice with this bacterium. The monoclonal antibody (225.4) reacts with the product of capsular polysaccharide 4 (CPS4) locus of B. thetaiotaomicron. Colonization of germfree Rag1-/- mice, with and without hydridoma cells that produce this antibody revealed that an engineered IgA response to this capsular epitope subsequently reduces pro-inflammatory signaling, suppresses epitope expression and impacts bacterial competitiveness. These finding indicate that 'tolerance'in the gut is a failure to develop pathological inflammation despite immune recognition of members of its microbiota. In Aim 1, I propose to use gnotobiotic mice to characterize additional antibodies with different epitope specificities than 225.4, as well as antibodies of different isotypes, and define, using functional genomic analyses of both the microbe and host, how each antibody, and various combinations of antibodies impact host-symbiont homeostasis. In Aim 2, I will develop an anti-symbiotic B cell receptor gnotobiotic transgenic-knock-in mouse to evaluate the development and long-term impact of a defined antibody response on host-symbiont homeostasis. Relevance: Understanding how we can co-exist with and benefit from the trillions of bacteria that reside in our digestive tract is important for understanding our health, and the origins of various diseases, including infectious diarrheas and colitis. I will evaluate how a part of the immune response, called immunoglobulin A, prevents inflammation. Understanding how this response is normally mounted and how it functions will allow the development of diagnostic test and treatments of inflammation mediated diseases in the future.